1. Field of Invention
The invention relates to an apparatus for controlling a hybrid vehicle (HEV) having two different drive units that supply a driving force to a transmission shaft that is connected to driving wheels.
2. Description of Related Art
In recent years, various types of hybrid vehicles have been proposed in an attempt to reduce adverse environmental effects and improve fuel economy. In a known system of such a hybrid vehicle (see, for example, Japanese Patent Publication No. 2000-002327), output from an engine and output from an electric motor are combined by a combining-distribution mechanism, and the combined output is transmitted to an input shaft through a ring gear or the like. Moreover, a synchronous coupling mechanism is controlled such that the gear ratio between the input shaft and an output shaft can be switched. In this hybrid vehicle, the synchronous coupling mechanism is operated as necessary to selectively couple (i) a low speed drive gear and the input shaft or (ii) a high speed drive gear and the input shaft, in order to switch a gear ratio between two speed stages, namely, a low speed and a high speed.
However, in the hybrid vehicle disclosed in Japanese Patent Publication No. 2000-002327, the output of the engine and the output of the electric motor are both transmitted to a differential through a transmission. Therefore, when the gear ratio of the transmission is switched by controlling the synchronous coupling mechanism, there is a transient delay in transmitting torque to the driving wheels. As a result, the driver may feel a sense of discomfort.
In an attempt to solve this problem, Japanese Patent Publication No. 2000-225578 discloses a hybrid vehicle having a first motor generator as a generator (hereinafter simply referred to as the “motor generator”); a second motor generator as a driving source (hereinafter simply referred to as the “drive motor”); a planetary gear mechanism for distributing engine output to the motor generator and the driving wheels, and for transmitting at least one of the output of the engine and the output of the drive motor to the driving wheels; and a power transmission state control apparatus for switching the output of the drive motor between a low speed state and a high speed state. According to this system of the hybrid vehicle, the power transmission state control apparatus is provided in a transmission path that is different from the path for transmitting the engine output to the driving wheels. As a result, unlike the previously described hybrid systems, the driver is not likely to feel any sense of discomfort when shifting takes place.
Specifically, in the hybrid vehicle disclosed in Japanese Patent publication No. 2000-225578, switching of the gear engagement state between the low speed state and the high speed state is performed by a stepped transmission that is provided further down the power transmission flow than the power transmission state control apparatus. Thus, even if there is a transient delay in transmitting the output of the drive motor to the driving wheels, the output of the engine is continuously transmitted to the driving wheels via a transmission path that is different from that of the power transmission state apparatus. Thus, a temporary drop in the driving force to the driving wheels is prevented, and the driver does not feel a sense of discomfort when shifting takes places.